Discordant expression of terminal transferase and T cell receptor beta chain in fetal and pediatric thymocytes

Human thymuses at different ages of development were analyzed for TdT+/beta F1+ double-stained cells. beta F1 is a mAb which recognizes a "hidden" framework determinant on the beta chain of the T cell receptor (TCR). We have found that TCR beta chains appear early during thymic ontogenesis and are detectable by 15 weeks of gestation in cells that are TdT-. Paradoxically, the immature CD2- large thymic blasts in late fetal development and in infants are TdT+ but beta chain negative. These data are compatible with the notion that TdT acts as somatic mutagen on TCR beta genes starting only after 20 weeks of gestation. The beta chain proteins which appear early during thymic ontogenesis might reflect expression of incompletely rearranged (DJ), unrearranged, or fully rearranged TCR beta genes in an early wave of thymocyte clonal (beta) diversification, that is, in contrast to later development, independent of TdT.